Kinetic parameters of CO adsorbed on Pt(111) studied by in situ high resolution x-ray photoelectron spectroscopy

Abstract

The kinetics of the adsorption system CO/Pt(111) has been studied by time-resolved high-resolution x-ray photoelectron spectroscopy using third generation synchrotron radiation at BESSY II. CO is dosed by a supersonic molecular beam device which allows for a high sample pressure (here up to 10−6 mbar) and a fast switching of the pressure. The site-specific occupation of CO adsorbed on on-top and bridge sites is determined quantitatively from C 1s spectra, which can be taken with a minimum collection time of 1.5 s per spectrum. Based on the observation of thermal equilibrium between the two sites, we perform a phenomenological analysis of the data, assuming a constant binding energy difference ΔE. From the on-top/bridge occupation ratio as a function of coverage obtained by uptake measurements we extract a value of ΔE=41 meV. With the same ansatz, ΔE is calculated from temperature-dependent measurements at a constant coverage. Finally, determination of the coverage during isothermal desorption is used to obtain the total binding energy of CO on Pt(111). Differences between our evaluation methods and literature reports are discussed.

Ultrafast x-ray photoelectron spectroscopy in the microsecond time domain

“…Since the adsorption energy difference between the on-top site and the bridge site is very small (40-90 meV, depending on the model 4,32 ), site exchange between these two sites is in equilibrium. Considering that the adsorption energy, E a , is much higher (e.g., 1.43 eV according to Ref.…”

“…When using synchrotron radiation, the time resolution is significantly improved, such that high-resolution XP spectra can be taken in a few seconds. 4 With this improvement it became possible to study time-dependent surface processes in situ, i.e., to follow adsorption as function of time or reactions as function of temperature. This gain in time resolution not only opens the possibility to access the kinetics of surface reactions, 5,6 but also to greatly improve the output in measurement data per time.…”

“…christian.papp@fau.de the growth of different adsorbate layers, 4,16 including (metal) oxides, [17][18][19] graphene, 20,21 and small hydrocarbons. [21][22][23][24] In addition to the mentioned applications, which all make use of synchrotron radiation in the "multi-bunch mode," for pump and probe-type experiments with much shorter timescales in the nanosecond regime or below the synchrotron facility must be operated in the single bunch mode, i.e., the pulsed time structure of the synchrotron radiation is made use of Ref.…”

“…We study the adsorption and transient desorption of CO at temperatures between 460 and 500 K, and compare the results to data obtained at lower temperatures with a time resolution of ∼4 s. Please note that the isothermal adsorption/desorption of CO on Pt(111) was up to now only studied up to temperatures of 450 K, as at higher temperatures the desorption is too fast, not allowing the required number of data points before complete desorption. 4 One specific advantage of the approach proposed here is its high flexibility, since the original electron analyzer setup is conserved, and only commercially available standard components are added, which makes the implementation highly cost effective.…”

CO-Induced Photoemission Structures of the CO/Pt/Ru(0001) Interface

“…The saturated layer is a mixture of on-top and bridge sites with a 1:1 ratio [10,11]. At 300 K, LEED shows the c(4 × 2) structure corresponding to the 0.50 SML at saturation with coexistence of two inequivalent sites [12].…”